Biopotential reading systems are used to electronically acquire electric signals from human body, such as heart signals as an electrocardiogram (ECG) and brain activity as an electroencephalogram (EEG). These systems can be used in any biopotential acquisition system, not only as ECG or EEG, but also in electromyography (EMG). They are also needed in different applications, including Body Area Networks (BANs). However, to be embedded in sensor nodes for such a power constrained application as for BANs, special techniques should be employed to meet special requirements such as relating to the power.
An ECG illustrates the electrical activity of the heart over time. Analysis of the various waves and normal vectors of depolarization and depolarization yields important diagnostic information. An EEG represents electrical signals from a large number of neurons and thus illustrates the electrical activity of the brain. EMG is a technique for evaluating and recording physiologic properties of muscles at rest and while contracting. EMG is performed using an instrument called an electromyograph that detects the electrical potential generated by muscle cells when these cells contract, and also when the cells are at rest.
There are two major issues in the design of front-ends for these systems: firstly, dealing with a differential DC offset superimposed on a much smaller useful signal and secondly, obtaining a high common-mode rejection. The first effect is due to electrochemical potentials at the interface between the skin and the electrodes used to sense the signals; the latter characteristic is required in order to reject strong interferers such as main signals coupled to human body.
The DC offset or DC component, respectively, in the differential signal introduces two major problems. Firstly, if only a low supply is available, it is not possible to amplify the differential signal before the DC is eliminated. As this DC offset Vdm,DC can be as high as ±300 mV, amplification by a factor of four will lead to the requirement of a voltage supply higher than 1.2 V.
Secondly, the presence of a high DC component brings an unbalance in the operation point of any circuit in the signal path. Thus, if a symmetrical circuit is used to reduce the effect of the common-mode, the bias point of the two halves of the circuit will be different due to the DC difference of the input signal. As it will be clarified in the following, this unbalance degrades the common-mode rejection. This unbalance cannot be eliminated by the usual techniques used to improve the common-mode rejection ratio (CMRR), as chopping. The reason is, that the unbalance is embedded in the differential signal and it would be chopped with it. This problem is described, for example, in R. F. Yazicioglu, P. Merken, R. Puers, and C. V. Hoof, “A 60 μW 60 nV/√Hz readout front-end for portable biopotential acquisition systems,” in ISSCC, February 2006, pp. 56-57. The CMRR of a differential amplifier measures the tendency of the device to reject input signals common to both input leads. A high CMRR is important in applications where the signal of interest is represented by a small voltage difference between two (possibly large) voltages. Accordingly, CMRR is a very important specification as it indicates how much of the common-mode signal will appear in the measurement. The value of the CMRR often depends on signal frequency as well, and can be specified as a function thereof CMRR is important in systems where noise is coupled in the same manner on both input leads of a differential circuit. It is very common in case of electromagnetic interference from the main or nearby electronic equipment.
Common solutions are disclosed, for example, in K. A. Ng and P. K. Chan, “A CMOS analog front-end IC for portable EEG/ECG monitoring applications,” in IEEE Transaction on Circuits and Systems-I, vol. 52, no. 11, pp. 2335-2347, November 2005, and R. Martins, S. Selberherr, and F. A. Vaz, “A CMOS IC for portable EEG acquisition system,” in IEEE Transaction on Instrumentation and Measurement, vol. 47, no. 5, pp. 1191-1196, October 1998. These solutions make use of high pass filtering of the signal. As the differential DC component Vdm,DC at the input contains no information but it is due only to parasitic effects, it can be eliminated without loss of information content. The required filter should have a high pass response with a lower cut-off smaller than the lower bandwidth limit of an ECG signal, i.e. 0.05 Hz. If very particular techniques are not used, the very low frequency singularities of the filter bring problems of integrability. Another disadvantage of the filter solution is a very slow startup and slow response to abrupt variations of Vdm,DC caused for example by motional artifacts. The latter problem may be overcome by storing the DC circuit conditions before the disturbance and resetting them automatically after the disturbance. For the startup this approach is invalid as the value of the signal DC offset is unknown and waiting times of more than 20 seconds are introduced. This can be an excessive overhead if the system is used for sensing the heartbeats for only a few minutes. A second possibility is the implementation of a high-resolution analog-digital-converter (ADC), which converts both DC and AC components of the input signal and directly shifts the filtering problem to the cascaded digital logic. However, such circuitry is not optimized in terms of power, since the DC offset is treated as useful signal, increasing the required resolution and the power consumption.